A naturally commutated cycloconverter is a static device for changing electrical power from one frequency to another. An inherent characteristic of such a cycloconverter is that the displacement angle of the current drawn at the input is lagging and is a function of the displacement angle of the output current. This characteristic is often acceptable, but for some applications it would be beneficial if the reactive current at the input could be controlled independently of the amplitude and phase of the current at the output.
This has been accomplished in the prior art, such as shown by U.S. Pat. No. 3,742,336, by connecting an external variable reactive power device across the input terminals and including back-to-back thyristors which are phase controlled to provide whatever net reactive current is required at the input, such as disclosed in the published article appearing in POWER for April 1973, at pages 69-71, irrespective of the output loading conditions of the cycloconverter. A fixed capacitor provides a fixed amount of leading reactive power from which is cancelled a lesser or greater extent of lagging reactive power drawn by the parallel connected variable inductor, when the cycloconverter experiences variable load conditions.
There has been in recent years an increased demand for power factor correction and control in utility and industrial power systems due to growing use of electrical machines, the major role of the electric arc furnace in steel production and the general acceptance of thyristor drives and power controllers in the industry. As a result, controlled generation of reactive power for improving the line power factor has become of major importance. This can be achieved most successfully through the use of static power switches, for instance thyristors. Traditionally, rotating synchronous condensers have been used for this purpose. It has been established, however, that static VAR generators in most applications provide superior performance at lower cost than conventional rotating synchronous condensers.
There are three basic modern methods of generating reactive power (VAR) all using static control of thyristors: (1) thyristor controlled shunt capacitors and inductors; (2) AC/DC converters and inverters; (3) AC/AC frequency changers.
The present invention relates to the third category of static VAR generators, namely to AC/AC frequency changers used for the generation of reactive power.
The AC/AC frequency changer is itself divided into several categories according to the mode of control, the range of control, and the inherent properties of the apparatus in operation. A basic distinction is made between frequency changers in which the thyristors are force commutated and those in which the thyristors are naturally commutated by the voltages of the input source.
The present invention relates to naturally commutated AC/AC frequency changers or cycloconverters.
Two inherent characteristics of a frequency changer using thyristors for conversion are: (1) the frequency relation between the input alternating current and the output alternating current; and (2) the phase relation between current and voltage at the input and at the output, i.e., the relationship between the input and output displacement power factors. In naturally commutated cycloconverters the input displacement power factor is lagging and is a function of the displacement power factor of the output current. It is possible by force commutation to control the input displacement power factor and in particular, to bring it automatically to unity. This is not possible with a naturally commutated frequency changer. Another particularity of frequency changers is that an alternating sinusoidal wave of a desired frequency is generated by controlled conduction of the thyristors. The time and frequency of conduction of the thyristors is generally variable along the reference waveform used to build the output waveform.
Static frequency changers offer a unique mode of generating reactive power. In that respect they provide an interesting alternative for AC/DC converters, and for inverters, which also are capable under proper operative conditions of generating reactive power. In all such instances, practically the reactive power generator must be operated in an essentially balanced multi-phase system. However, these types of generators not only perform generally as well as any rotating synchronous condensers under steady state conditions, but also have proved to be superior for transient response.
In the U.S. Pat. No. 3,858,105 of Laszlo Gyugyi reference has been made to an original concept by B. R. Pelly of a high frequency link consisting of a cycloconverter and a high frequency source of reactive power naturally commutating the cycloconverter for providing, in relation to a power system connected at the output of the cycloconverter, power factor correction by control of the cycloconverter.
In the above-mentioned Gyugyi patent it is also stated that the cycloconverter has at the input an inherent lagging component of current, similar to that drawn by an inductance. This lagging input current varies with the output load, so that the frequency of the reactive power source provided by a tuned L-C circuit, required for natural commutation of the cycloconverter, varies. Therefore, a higher VA rating is required from the L-C tuned circuit reactive source in order to keep its frequency variations small under varying load conditions. In the patent, the proposition is made to use two cycloconverters properly controlled to create a system equivalent to one cycloconverter in which the load variations do not affect the lagging input power factor exhibited by the two cycloconverters. The present invention proposes to achieve a similar result by establishing and controlling a circulating current between banks of one or two cycloconverters.